When a drilling bit is rotating on the bottom of a well bore, it is constantly bouncing up and down. A commonly accepted explanation for this action is that the threecone bit forms three lobes on the bottom of the well bore - as the bit moves over these lobes, it is axially displaced three times during each rotation.
Acceleration of the bit off bottom causes high loading of the drilling string. More particularly, the bit has a load on it arising from the weight of the drilling string. For example, the string might weight 120,000 pounds, of which 60,000 pounds might be held suspended by the rig; the remaining 60,000 pounds would bear on the bit. When the loaded bit is accelerated off bottom through a travel of perhaps 1/2 inch to 1 inch, the essentially rigid drilling string above the bit is subjected to a very high shock load, which is immediately relieved as the bit begins to return to bottom. By way of example, this cyclic loading on the drilling string may vary between 0 and 100,000 pounds or more from one moment to the next.
There are several deleterious effects which arise from the severe cyclic loading to which the drilling string is subjected. For example, it is a prime factor in the wear and failure of the drilling string. It also punishes the rig; particularly rough drilling, the whole rig structure is shaken violently and the only course of action available to relieve the vibration is to reduce the rotational speed of and/or weight on the bit. This causes a reduction in the drilling rate.
It has long been common in the industry to insert a tool, known as a vibration damper or shock absorber, in the drilling string above the bit with the aim of isolating the string from the bit.
In general, a typical vibration damper would comprise an inner, tubular mandrel, attached at its upper end to the drilling string, and an outer, tubular barrel attached at its lower end to the bit, or the collars which are directly above the bit. The mandrel slides or telescopes within the barrel. The two parts are connected by means, such as a spline assembly, so that they are locked for rotation together but can move longitudinally relative to each other. Means are also provided to limit the extent of longitudinal movement of the parts so that they cannot separate one from the other. In one basic type of tool, a portion of the mandrel is reduced in outside diameter so that an annular chamber is formed between the mandrel and barrel. O-ring seals are provided between the mandrel and barrel at each end of the chamber to prevent drilling mud from entering therein. The mandrel and barrel carry opposed upper and lower compression shoulders respectively; these shoulders extend transversely into the annular chamber adjacent its upper and lower ends. A deformable element is provided within the chamber between the compression shoulders.
In operation, when the bit is accelerated upwardly, the barrel compression shoulder acts against the base of the deformable element. The element is prevented from moving axially by the mandrel compression shoulder located at its other end. As the shoulders squeeze together, the element is deformed. In theory, the deformable element should absorb the axial thrust of the bit and prevent the shock load from being transmitted to the drilling string. In practise, this is usually not the case, for reasons which will now be discussed.
The prior art tools can be divided into three types.
In the first type, the O-ring seals are fixed in the wall of the barrel at each end of the deformable element chamber. As a result, the well bore hydrostatic pressure acting on the tool forces the barrel upwardly against the deformable element with a force equal to said pressure times the difference in cross-sectional area of the two seals. To try to cope with this "pre-loading" action which takes place when the tool is in the well bore but before drilling commences, it is conventional to use a "hard" deformable element in the chamber. By a hard element is meant an element having a spring rate of at least 100,000 pounds/inch, usually in the order of 150,000 - 250,000 pounds/inch, where the spring rate is described as the load required to deflect the tool an inch. In these prior art tools, the tool may telescope 3/4 inches with a load of 100,000 pounds, but will telescope less than 1/4 inch with the next 100,000 pounds. In effect, the tool becomes extremely hard or rigid as the load increases. According to our calculations, in deep wells the deformable element in this type of tool will have lost any shock absorbing capability it had by the time that the tool is at total depth, even before drilling commences. For example, if one were to consider a 12,000 foot deep well bore containing a tool having a differential in seal area of 30 inches, the upward thrust on the barrel created by the hydrostatic pressure could be in the order of 300,000 pounds. In this circumstance, the deformable element would essentially be rigid and ineffective since the tool would have collapsed through most of its stroke. It is evident that in these tools, an element that is relatively soft at the surface would be extremely hard at operating depth due to the very high "pre-load" that it would carry.
The second type of tool is disclosed in Canadian Pat. No. 837,970, issued to Faulkner. In this tool, a floating seal is provided at the base of the deformable element chamber to equalize the pressure internal of the chamber with the bottom hole hydrostatic pressure. However, Faulkner teaches combining this feature with compressible, metal wire elements which are hard elements to begin with and which rapidly pack in use and form virtually non-deformable elements having little shock-absorbing capability. The element taught by Faulkner also has the disadvantage that it is in contact with both the walls of the chamber, thus tending to prevent axial movement of the telescoping tool parts, as the element packs in the chamber.
The third type of tool attempts to provide a relatively "soft" element in the chamber, i.e. one having a low spring rate, and couples it with means for equalizing the pressure within the chamber with the well bore pressure. This tool is described in Canadian Pat. No. 826529, issued to Galle. It involves providing a sealed zone, filled at surface with compressible gas at a pre-determined pressure, in the chamber. The chamber is filled with operating oil and a membrane is provided to segregate the gas from the operating oil. A bag, or membrane, open to the well bore, extends into the oil-filled section of the chamber. The chamber is sealed at each end with fixed O-ring seals. Expansion of the bag with the well bore fluid pressurizes the oil and, in turn, the shock-absorbing, compressible gas. However, the pressure and thus the spring rate of the gas body must be varied significantly as the tool is used at different depths. Published reports show that at a depth of 16,000 feet and a bit weight of 80,000 pounds, the spring rate of this device is about 140,000 pounds per inch, while at a depth of 3,000 feet at the same bit weight, the spring rate is about 60,000 pounds per inch. Therefore at the deeper depth, this tool has a "hard" element with a high spring rate, while at the shallow depth, the element can be classed as being moderately soft with a moderate spring rate.
A characteristic of most shock absorbing elements which rely on deformation of a material to absorb shock is that the spring rate of the element increases as the load on the element is increased. In other words, with low loading on an element, the element is much softer than with high loading. The graph illustrated in FIG. 15 shows the deflection-load characteristics in curve form for three elements A, B, and C. As is evident from the graph, the spring rate of the elements (i.e. the slope of the curve) varies continuously as the load on the element changes. Consequently, we must refer to the spring rate as being an average figure for a certain amount of deflection. We describe the spring rate as the load required to deflect the tool one inch.
Element A of the graph can be classified as being hard, having a spring rate for its first inch of travel of about 300,000 pounds per inch. If, at operating depth, the element was pre-loaded, (operating point X on the graph), with 300,000 pounds as described earlier, it is evident that the element would be extremely hard.
Element B of the graph can be classified as being moderate, having a spring rate of 55,000 pounds per inch for its first inch of travel. If at operating depth, the element was pre-loaded (operating point X on the graph) to 300,000 pounds, it is evident that the element would be extremely hard, i.e. it would take an enormous load to further deform the element. If at operating depth, the element was only pre-loaded to 30,000 pounds (operating point Y on the graph), then the element would be moderately soft, having a spring rate of about 60,000 pounds per inch. It could be further deformed, with a relatively small load.
Element C of the graph can be classified as being soft, having a spring rate of 15,000 pounds per inch for its first inch of travel. At operating point X on the graph, the spring rate would be very high and the element would be extremely hard. However, at operating point Y, the element would have a spring rate of about 20,000 pounds per inch.